The present invention relates to pebble-bed type high temperature gas reactors, and more particularly to gas reactors having a multi-cavity prestressed concrete reactor vessel (PCRV).
Pebble-bed core nuclear reactors utilize a core comprising a column of spherical fuel elements, each of which is approximately 6 cm in diameter. One advantage of pebble-bed core reactors is that they allow fuel to be exchanged during the operation of the reactor. Another is that the spherical fuel has a high rate of burn.
There are two types of pebble-bed core reactor designs: monolithic designs consisting of a single large core, and module designs which utilize a number of smaller cores.
Monolithic pebble-bed reactors have a large core disposed at the center of a prestressed concrete pressure vessel. Primary cooling systems such as gas circulators and steam generators are located around the core. The monolithic design has the advantage that a single unit can produce high output. Currently, a 300 megawatt reactor of monolithic design is in operation in Germany and a 500 megawatt plant is being developed. There are, however, disadvantages to this design. For instance, due to the size of the core, low temperature shut-down can not be achieved by merely inserting control elements into the reflector regions which surround the core. Instead control elements must be inserted directly into the core. This can result in damage to the spherical fuel elements. Another disadvantage of the large cores associated with reactors of the monolithic design is that they require large reflectors above the core. Large reflectors are difficult to support and are prone to damage from earthquakes.
Module type pebble-bed reactors consist of a number of smaller cores and primary cooling systems. Each core and its associated primary cooling system is housed in a separate individual steel pressure vessel which is connected by a large bore pipe to its associated primary cooling pressure vessel to form a single unit.
The diameter of the core of modular type reactors typically is no greater than 3 meters, and they have a power density of 3 watts per cubic centimeter or less. Low temperature shut-down of a module type core can be achieved by merely inserting control elements into the reflectors which surround the core, making it unnecessary to insert control elements directly into the core and risk damage to the fuel pellets. Furthermore, due to their small size, the cores of module type reactors do not need to support large reflectors over the core, making the core less prone to earthquake damage.
The output of a single module type pebble-bed reactor being about 80 MW, a number of them must be used in order to produce an output equal to that of a monolithic style reactor. Facilities comprised of enough modular type reactor units to produce an output equivalent to that of a monolithic reactor generally require much larger sites than the facilities associated with the monolithic reactors. Thus, module type reactors are far less efficient in terms of site utilization. Another disadvantage of the modular type of design is that the pipe which connects each core pressure vessel to its primary cooling system is subject to damage from earthquakes and similar disasters.
In an earlier design proposed by one of the present applicants and another, described in copending U.S. patent application Ser. No. 07/475,693, filed Feb. 6, 1990, the reactor core region is surrounded by a unitary reflector which is sectioned into a plurality of core sub-regions (typically three) by means of partition walls formed of reflector material, and the unitary reflector is housed within a single-cavity prestressed concrete pressure vessel. The primary cooling systems, one for each core sub-region, are disposed in the annular space between the reflector and the inner wall of the pressure vessel. A disadvantage of this design is that because of the proximity of the core sub-regions and the fact that the cores are separated from each other only by reflector material, typically graphite, conditions in one of the reactors can influence the condition of its neighbors. That is to say, an accident in one core can cause damage to one or more other cores.
It is an object of this invention to provide a pebble-bed type high temperature gas reactor which can produce up to several hundred megawatts of output without suffering the described disadvantages of previous designs.